Determine the size of a binary file

Question

I'm trying to read a binary file and I need to determine its size, but regardless of the method I've tried, I'm getting a size of zero.

For example:

fstream cbf(address, ios::binary | ios::in | ios::ate);
fstream::pos_type size = cbf.tellg();                   // Returns 0.

char* chunk = new char[size];
cbf.read(chunk, size);
//...

If I were to use the following:

#include <sys/stat.h>
struct stat st;
stat(address.c_str(),&st);
int size = st.st_size;

The size is still zero. I've also tried the following, but it's still zero.

File* fp;
fp = open(address.c_str(), "rb");

How do I get the size of the file?

Thanks for the responses... I've identified the problem: The binary file I was trying to access was created during the execution, and I just had forgotten to close it before trying to read from it...

Solution

Neither of your examples checks for failure. This program, using your first method, works perfectly well for me. It correctly identifies the size of /etc/passwd and the non-existence of /etc/motd.

#include <fstream>
#include <iostream>
#include <string>

void printSize(const std::string& address) {
  std::fstream motd(address.c_str(), std::ios::binary|std::ios::in|std::ios::ate);
  if(motd) {
    std::fstream::pos_type size = motd.tellg();
    std::cout << address << " " << size << "\n";
  } else {
    perror(address.c_str());
  }
}

int main () {
    printSize("/etc/motd");
    printSize("/etc/passwd");
}

Comments

[matth]

Welcome to SO! Remember to check the return value from system calls. You don't check to see if stat or fstream::open succeed. You should not rely upon their results until and unless you know that they succeeded.

[Ash]

i do that in my code...i check for fail() , good() , is_open() etc...i just didn't want to bother you with unnecessary code...sorry

[matth]

It is better to bother us with complete code then with sorta-kinda code. We can't know what your program does if you don't tell us. Please reduce your original program to the smallest possible program that still demonstrates the error and paste that complete, minimal program into your question. See sscce.org for more details.

[Ash]

wow you edited my whole post...didn't realise my english was so bad!

[danial weaber]

use ifstream cbf to open the file. else look my answer below.

[matth]

@Ash, what output do you get when you run my program with no changes?

[Michael Burr]

@Ash - are you passing an absolute path? Are you running inside an IDE where the cwd might not be what you think it is? Can you post the output of ls -al [filename] where [filename] is the exact string you pass to printSize()?

[Ash]

@Michael Burr: that's what i get -rw-rw-r-- 1 ash ash 6 nov. 30 16:44 compressed_string

[Ash]

@Rob: with no change : 113 1768 and for my own file 6 well that's exactly the size of my file so thanks i guess the problem comes fom the changes i make^^

[Ash]

@Rob : but still when i use your function(without change) in my own code it returns 0 instead of 6 !!

[matth]

Can you expand upon why you think using ifstream instead of fstream would make a difference?

[Ash]

thanks but it's just like all the other suggestions,it works perfectly fine when it's isolated , once i use it in my class functions it starts returning 0 instead of the real size of the file ...

[danial weaber]

@Robᵩ :) I dont know that man, Im not a expert. this is just how I was taught and this work fine. sometimes this will work grate for ash.

[Michael Burr]

@Ash: are you sure about the cwd when you're running the program? Is there another compressed_string file that has zero size that the program is really dealing with?